1. Field of the Invention
This invention relates to the field of color transformation, and specifically relates to a color management system that preserves black content when transforming from a source device color space to a destination device color space, while accommodating user-selection of a gamut-mapping model for the transformation.
2. Description of the Related Art
Color management systems are used for transforming color image data from the color space of a source device to the color space of a destination device. Generally, in color management systems such as the system of the International Color Consortium (ICC), the component colors of source color data are first transformed from a device-dependent color space for the source device into a device-independent profile connection space (PCS), such as CIELAB or CIEXYZ color space. The color data is then transformed from the PCS color space into a device dependent color space for the destination device.
Many times, particularly in color proofing operations, it is desirable to transform color image data from a source color space that includes a black colorant value to a destination color space that also includes a black colorant value, such as from the color space of one type of color printer to the color space of another type of color printer. Examples of such color spaces include CMYK, which is a four-component color space comprising cyan, magenta, yellow and black, and CMYKOG, which is a six-component space that also includes orange and green. When converting colors from such a source device to a destination device, it is often desirable to preserve the black component (K) such that the amount of black present in the source image is pleasingly reflected in the destination image.
An important aspect of black preservation is the ability to preserve black-only content, i.e. the data within an image composed solely of black, or purely neutral gray levels between white and black. For example, a portion of a source image may be comprised only of black or gray text or lines, which is represented in the source CMYK data using only the K component. If the black-only content is not preserved, the corresponding portion in the destination image is represented as a combination of C, M and/or Y components, and not just the K component. However, the combination necessary to achieve a pleasingly acceptable black appearance is very delicate, and if the combination is not correct, a brownish, muddy appearance can result. Furthermore, the printing of black text and/or other objects with fine detail using multiple colors of inks can result in blurred text and/or objects due to slight errors in alignment while printing. Accordingly, it is desirable to preserve the black-only content of the source image for accurate representation in the corresponding destination image.
In addition, some color management systems contain a user-selectable gamut-mapping model, which allows a user to select a particular gamut-mapping algorithm. In such systems, the user selects the algorithm for transforming source color data to destination color data that might otherwise map to an out-of-gamut color in the destination color space.
However, when the user selects the gamut-mapping algorithm, he/she also expects that black-only content is preserved, regardless of the gamut-mapping algorithm that is selected. Although prior attempts to preserve black-only content have been made, none are seen to address the problem of preserving black-only content in a color management system with user-selectable gamut-mapping.